The present invention relates to electrical signal-carrying cables, and particularly to controlled-impedance cables which may be immersed in fluids having dielectric constants different from that of air, and which may include shield conductors.
A significant problem in the use of signal transmission lines in the form of coaxial cables or jacketed or shielded insulated sets of conductors in applications where the characteristic impedance and signal transmission speed are critical is that a change in the dielectric constant and thus the characteristic impedance and transmission speed of the transmission line occurs if the transmission line is immersed in a fluid other than air, such as a coolant. In some applications of transmission lines a coolant is necessary because of the amount of power dissipated during operation, but the change in impedance and speed of such transmission lines which occurs as a result of immersion of the transmission lines in a coolant is so significant that some devices, such as computers, in which the transmission lines are used cannot be operated or even tested after immersion in the coolant until the change in impedance and speed of the transmission line has occurred and conditions have become stabilized.
It is therefore important for transmission lines to be used in such applications to become stabilized as rapidly as possible, in order for testing to be accomplished after repairs have been made. Extruded jackets for transmission lines require at least several hours to provide for replacement of air found in the interstices within the jackets, between the conductors and between the conductors and jacket elements of such transmission lines. In the case of very large computers the value of time lost waiting for stabilization of transmission lines can be tremendous.
One solution to this problem has been the use of some types of expanded polytetrafluoroethylene (PTFE) as the material for the jacket layer surrounding the conductors of such a transmission line. The porosity of such expanded PTFE permits a rather rapid entry of coolant fluid to displace the air otherwise contained within the jacket when the transmission line is not immersed in a cooling fluid.
While such construction of transmission lines enhances the stabilization of impedance in response to immersion of the transmission line in a cooling fluid having a different dielectric constant than that of air, electrical connection to the shield conductor of such transmission lines has previously required that the shield conductor be uncovered to provide a location for interconnection of the shield conductor to a ground lead or to another conductor which forms part of an electrical circuit incorporating the transmission line.
What is needed, then, is an improved structure for an electrical cable, and particularly for a high-speed signal transmission line, which permits rapid infiltration of a dielectric fluid into the interior of such an electrical cable, so that changing impedance and speed resulting from immersion of the cable into a fluid such as a coolant will not unduly delay testing and operation. It is also desired to have such a transmission line or similar electrical cable which can be connected more easily and quickly than has previously been possible.